Data transmission systems or networks use some type of medium access control protocol to control access to the physical medium, e.g., an AC power line or Ethernet cable, if the medium is a shared medium. That shared medium access mechanism may be polling, Time Division Multiple Access (TDMA), token passing, Carrier Sense Multiple Access (CSMA), or some other shared access protocol. Polling uses a centrally assigned master station to periodically poll other (slave) stations, giving those other nodes explicit permission to transmit on the medium. In the TDMA protocol, a network master broadcasts a frame synchronization signal before each round of messages to synchronize clocks of all stations and, after synchronization occurs, each station transmits during a uniquely allocated time slice. In the token passing scheme, access to the transmission medium is determined by possession of a special data unit called a token, which is passed from station to station. In the CSMA protocol, all transmissions are broadcast on the medium and stations listen to the medium to determine when the medium is idle before transmitting. In CSMA with collision avoidance (CSMA/CA), each station listens to the medium while each transmission is in progress and, after the transmission ends, waits a specified interval (or interframe gap) followed by an additional delay of one or more transmission (or contention resolution) slots based on a selected slot number before transmitting.
There are variations on one or more of these protocols related to prioritization, which may be necessary to ensure efficient station-to-station dialog or Quality of Service (QoS) requirements. For example, with CSMA/CA, the first slot may be reserved for a response by the station that just received a message, or, alternatively, one or more of the slots may be reserved for transmissions (or stations) of a particular priority class.
In some network environments, such as power line network environments, the CSMA protocol operation may be adversely affected by distances between stations and/or channel conditions. Additionally, the CSMA protocol operation may be susceptible to interference (e.g., collisions) caused by overlapping networks, that is, networks that are not intended to communicate with one another but can in fact hear each other's transmissions, as well as by a hidden station (or node). The hidden station is a station in a network that, because of its location, may hear only half of a communication exchange between other stations in the same or neighboring networks. Under such circumstances and conditions, the CSMA scheme may not function effectively to maintain proper network synchronization and orderly media access arbitration. Another consequence is that the network may be unable to ensure strict adherence to QoS guarantees.